Questions about equivalents of audio/video and digital/analog.

Floyd L. Davidson wrote:

(snip)

> In a digital channel you cannot pass frequencies higher
> 1/2 the Nyquist rate, which in theory is a very sharp
> cutoff but in practice it becomes very similar to the
> gradual analog cutoff.  

If you read Nyquist's paper, that is pretty much it.

He was figuring out for fast he could send pulses
through a band limited channel and separate them out at
the other end.  Electronic communication was digital
before it was analog.

-- glen

Jerry Avins <jya@ieee.org> wrote:
>Dave Platt wrote:
>> In article <1187572498.074750.50210@i38g2000prf.googlegroups.com>,
>> Radium  <glucegen1@gmail.com> wrote:
>>
>>> I'm curious to why there are no purely-analog devices which can
>>> record, store, and playback electric audio signals [AC currents at
...

>> The net result is that an audio CCD is capable of
>> storing a
>> decent-quality signal for only a few tens or hundreds of milliseconds,
>> from input to output.
>> Another sort of a purely analog signal-storage device,
>> with no moving
>> parts other than the electrons which convey the signal, is a simple
>> length of transmission line (with perhaps some amplifiers mid-way).

...

>Come on, Dave, a CCD is a digital device, subject to
>aliasing.

CCDs are analog devices, with an analog voltage output.

The fact that they are commonly used as the sensor in
digital cameras results in the output of a CCD virtually
always going directly (well, after a bit of signal
processing for things such as white balance, ISO gain,
etc.) to an analog-to-digital converter that digitizes
the analog signal.

>The charges represent the signal at a
>particular instant of its average over a particular
>interval. (My CCD digital camera can take time
>exposures.) A CCD's content may not be quantized in
>amount, but it is quantized in time. In a camera, where
>the charges pertain to individual pixels, the result is
>also quantized in space.

But none of that quantization changes the fact that the
device itself has an analog output.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

nospam@nospam.com (Don Pearce) wrote:
>On Sun, 19 Aug 2007 23:26:16 -0700, dplatt@radagast.org (Dave Platt)
>wrote:
>
>>"Digital" and "subject to aliasing" are two different things.
>>
>>As I believe the term "digital" is usually meant, it implies a
>>two-state (on/off) storage representation.  It's not just that the

That describes a binary digital system.  Not all digital systems
are binary.  What is called M-ary is very common, with multiple
states.

>>signal amplitude is quantized, but that the quantization uses a
>>power-of-two representation and storage system of some sort.

It doesn't require a power of two representation, though that
certainly makes a lot of other functionality much easier.  The
key is "discrete states" from a "finite set".  That makes it
digital.

>My reading of the possible systems goes like this.
>
>analogue - a continuous representation of the original signal
>sampled - a representation of the signal at discrete time points

Note that discrete time points does not make a signal digital,
if the value of the signal can still be varied infinitely.

>quantized - a sampled signal, but with the possible levels constrained
>to a limited set of values

That is by definition a digital siganl.  As soon as the possible values
are "constrained to a limited set", it is by definition digital data.

>digital - a quantized signal, with the individual levels represented
>by numbers

It makes no difference how the levels are represented.

>Aliasing is going to happen as soon as you move beyond the first line
>of that list.

Your definitions are pretty good!  The significant
points are that analog is continuous with an infinite
set of values, while digital has a discrete number of
values from a finite set.

The standard definitions of analog data and digital data
(these are milspec and Federal Standard 1037C
definitions) are:

 analog data:

    Data represented by a physical quantity that is
    considered to be continuously variable and has a
    magnitude directly proportional to the data or to a
    suitable function of the data.

 digital data:

    1. Data represented by discrete values or
    conditions, as opposed to analog data.

    2. Discrete representations of quantized values of
    variables, e.g. , the representation of numbers by
    digits, perhaps with special characters and the
    "space" character.

See http://glossary.its.bldrdoc.gov/fs-1037/

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

Jerry Avins <jya@ieee.org> wrote:
>I like your categories. It is possible in concept to
>have a signal that is quantized in magnitude and
>continuous in time, but (unless we resort to counting
>electrons) I don't think it's possible in practice.

If you quantize the magnitude, it is digital.  That is
by definition.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

Jerry Avins <jya@ieee.org> wrote:
>I like your categories. It is possible in concept to
>have a signal that is quantized in magnitude and
>continuous in time, but (unless we resort to counting
>electrons) I don't think it's possible in practice.

If you quantize the magnitude, it is digital.  That is
by definition.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

Jerry Avins <jya@ieee.org> wrote:
>I like your categories. It is possible in concept to
>have a signal that is quantized in magnitude and
>continuous in time, but (unless we resort to counting
>electrons) I don't think it's possible in practice.

If you quantize the magnitude, it is digital.  That is
by definition.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

Jerry Avins <jya@ieee.org> wrote:
>I like your categories. It is possible in concept to
>have a signal that is quantized in magnitude and
>continuous in time, but (unless we resort to counting
>electrons) I don't think it's possible in practice.

If you quantize the magnitude, it is digital.  That is
by definition.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

Jerry Avins <jya@ieee.org> wrote:
>I like your categories. It is possible in concept to
>have a signal that is quantized in magnitude and
>continuous in time, but (unless we resort to counting
>electrons) I don't think it's possible in practice.

If you quantize the magnitude, it is digital.  That is
by definition.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

Jerry Avins <jya@ieee.org> wrote:
>I like your categories. It is possible in concept to
>have a signal that is quantized in magnitude and
>continuous in time, but (unless we resort to counting
>electrons) I don't think it's possible in practice.

If you quantize the magnitude, it is digital.  That is
by definition.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

Jerry Avins <jya@ieee.org> wrote:
>I like your categories. It is possible in concept to
>have a signal that is quantized in magnitude and
>continuous in time, but (unless we resort to counting
>electrons) I don't think it's possible in practice.

If you quantize the magnitude, it is digital.  That is
by definition.

-- 
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com